As an electrophotographic image forming method, there is an image forming method comprising the step of developing an electrostatic latent image on an image carrier employing a two-component developer composed of toner and carrier to form a toner image. The carrier constituting this two-component developer is broadly divided into a conductive carrier composed of oxidized iron powder or nonoxidized iron powder, and an insulating carrier evenly covering an insulating resin on the surface of a core particle called a carrier core composed of a ferromagnetic material such as iron, nickel or such. Of these, the insulating carrier has an advantage of excellent durability and longer life over the conductive carrier, and is employed as a carrier suitable for high-speed image formation.
There is provided ferrite as raw material for a carrier core. Ferrite is α-iron having a body-centered cubic crystal, or a solid solution of a divalent transition metal such as Ni, Cu, Co, Mn, Zn or such into the α-iron. In this situation, the solid solution of a divalent transition metal such as Ni, Cu or such exhibits an excellent property as a carrier core, but there is a recent move to aim for product safety via removal of harmful substances to humans from business machines and home appliances. For example, Restriction of Hazardous Substances (RoHS) to ban using specific 6 substances for electrical and electronic products sold in EU is to be a typical example.
Since there is concern that Cu and Zn are accumulated in living organism though they are not RoHS regulation substances, solid solution type ferrite containing no such metals has been intensively studied, and as a result, attention is currently focused on Mg-containing ferrite.
There is the following carrier technique employing Mg-containing ferrite. For example, there is a method of producing MnO—MgO—Fe2O3 system soft ferrite by baking oxide raw material in the presence of a mixed gas formed from an oxygen gas and an inert gas (refer to Patent Document 1, for example).
Also provided is a technique of producing a carrier by coating a polymethylalkyl siloxane-added resin on the surface of a soft ferrite particle having a MnO—MgO—Fe2O3 composition (refer to Patent Document 2, for example).
Further, there is a technique of producing a carrier core by adding metal oxide such as Bi2O3 having a melting point of at most 1000° C. and metal oxide such as ZrO2 having a melting point of at least 1800° C. into Mg-containing ferrite (refer to Patent Document 3, for example).
In this way, the carrier technique employing Mg-containing ferrite has been studied.    (Patent Document 1) Japanese Patent O.P.I. Publication No. 2001-93720    (Patent Document 2) Japanese Patent O.P.I. Publication No. 2003-337445    (Patent Document 3) Japanese Patent O.P.I. Publication No. 2004-240321